Thunderbird has had hidden support (Maildir is disabled by default) for the Maildir mail storage format for several years but Maildir is still not 100% ready for users. This article explains the format change.

WARNING:Unless you are advanced and know how to backup your email before turning on Maildir and how to restore it if you run into problems, we suggest you leave Maildir disabled.

Maildir is a single file per email (EML) unlike the current default monolithic single file format which is one file per folder (known a mbox). This storage format has significant benefits:

Backups are easier because amount of data needed to be backed up is reduced to just those emails that have changed since the last backup.

It is easier for anti-virus to scan emails in individual files and infected mails can be removed without the loss of all the other mail in the folder as often occurs today with anti-virus software which cannot handle multiple emails in one file.

This is a major change for Thunderbird because it changes the mail storage mechanism. The development team have spent a significant amount of time developing maildir
.

However due to a lack of time, and testers, before Thunderbird 38 was released the actual underlying changes have not had the extensive manual testing that would normally occur for such a major change.

How to turn on Maildir

WARNING: Again, this feature is experimental and we recommend that you know how to file bugs and how to backup and restore your email before turning on Maildir.

Here's how to turn on Maildir:

Backup your email. If you don't know how to backup your email, then you are not qualified to test Maildir.

Click the Application menu button
.

Click Preferences and then OptionsPreferences.

Click Advanced and then click General.

Next to "Message Store Type for new accounts", select File per message, (maildir).

Any email account created from now on will use Maildir (existing accounts will continue to use mbox). Please file a bug if you encounter any Maildir issues.